Method and apparatus for recovering timing information in orthogonal frequency division multiplexing (OFDM) systems

ABSTRACT

An improved OFDM receiver is disclosed that repositions peaks in an OFDM frame to a desired position away from the frame boundary to reduce the-probability of timing ambiguity. Each OFDM frame is divided into at least two windows during an acquisition mode to identify the index within each window having the maximum correlation. The improved timing acquisition of the present invention permits the Fast Fourier Transform (FFT) operation to operate on the correctly aligned symbol for improved accuracy. In addition, the present invention provides improved mechanisms for declaring when timing is acquired or when timing has been lost. In one implementation, the peaks are shifted from the frame boundary to the center of the OFDM frame, thereby removing the ambiguity of whether a given peak is associated with a previous or subsequent frame. A timing FSM processes a number of timing estimates to determine when the timing information has been acquired and shifts the OFDM signal, as necessary, to maintain the peak in the desired position during a tracking mode. The timing FSM determines that timing acquisition is completed if a timing estimate does not shift by more than a guard interval for a predefined timing acquisition length. After a predefined inter-mode settling period, the timing FSM will transition to a tracking mode. If the differences between the estimated times and the desired position of the OFDM frame are consistently greater than the length of the guard interval, then a loss of tracking is achieved and the timing FSM will return to an acquisition mode.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present invention is related to U.S. patent application Ser.No. 09/398,502, filed Sep. 17, 1999, entitled “Method and Apparatus forPerforming Differential Modulation Over Frequency in an OrthogonalFrequency Division Multiplexing (OFDM) Communication System,” (AttorneyDocket Number Riazi 3-11-3), assigned to the assignee of the presentinvention and incorporated by reference herein.

FIELD OF THE INVENTION

[0002] The present invention relates to wireless communication systems,and more particularly, to methods and apparatus for recovering timingestimates in an orthogonal frequency division multiplexing (OFDM)communication system.

BACKGROUND OF THE INVENTION

[0003] Satellite broadcasting systems for transmitting programmingcontent have become increasingly popular in many parts of the world.Direct Broadcasting Satellite (DBS) systems transmit televisionprogramming content, for example, to a geo-stationary satellite, whichbroadcasts the content back to the customers. In such a wirelessbroadcast environment, the transmitted programming can be received byanyone with an appropriate receiver, such as an antenna or a satellitedish.

[0004] In addition, a number of satellite broadcasting systems have beenproposed or suggested for broadcasting audio programming content fromgeo-stationary satellites to customers in a large coverage area, such asthe continental United States. Proposed systems for providing digitalaudio broadcasting (DAB), for example, are expected to provide nearCD-quality audio, data services and more robust coverage than existinganalog FM transmissions. Satellite broadcasting systems for televisionand radio content provide potentially national coverage areas, and thusimprove over conventional terrestrial television stations and AM/FMradio stations that provide only regional coverage.

[0005] Satellite broadcasting systems transmit digital music and otherinformation from an uplink station to one or more mobile receivers.Satellite broadcasting systems typically include a plurality ofsatellites and terrestrial repeaters operating in a broadcast mode. Thesatellites are typically geo-stationary, and are located over a desiredgeographical coverage area. The terrestrial repeaters typically operatein dense urban areas, where the direct line of sight (LOS) between thesatellites and the mobile receiver can be blocked due to the angle ofelevation and shadowing by tall buildings.

[0006] Orthogonal frequency division multiplexing (OFDM) techniques havealso been proposed for use in such satellite broadcasting systems andother wireless networks. In an OFDM communication system, the digitalsignal is modulated to a plurality of small sub-carrier frequencies thatare then transmitted in parallel. It has been found that OFDMcommunication systems do not require complex equalizers, even at highdata rates and under multipath propagation conditions. Among otherbenefits, OFDM communication systems provide a guard interval thatabsorbs the multipath distortion into the guard interval duration. Aslong as the arrival times of the multipath signals differ from oneanother by less than the guard interval, an equalizer is not necessary.

[0007] An OFDM receiver must perform timing acquisition and tracking toprocess data properly. FIG. 1 illustrates portions of a conventionalOFDM receiver 100 directed to timing recovery. The OFDM receiver 100implements a known Guard Interval Based (GIB) algorithm 110 thatrecovers timing information from the received signal. For a moredetailed discussion of the GIB timing recovery algorithm 110, see, forexample, Jan-Jaap van de Beek et al., ML Estimation of Time andFrequency Offset in OFDM Systems, IEEE Transactions on SignalProcessing, Vol. 45, No 7, 1800-05 (July 1997) or Jan-Jaap van de Beeket al., “A Time and Frequency Synchronization Scheme for MultiuserOFDM,” IEEE J. on Selected Areas in Communications, Vol. 17, No. 11,1900-14, (November 1999), each incorporated by reference herein.

[0008] Generally, the GIB timing recovery algorithm 110 employed by theOFDM receiver 100 identifies peaks in the maximum likelihood (ML) metric200, shown in FIG. 2. Each peak, such as the peaks 210-216, in the MLmetric 200 corresponds to the start of each OFDM frame. The peaks arepresent because the received samples are heavily correlated at a lagcorresponding to the useful symbol duration. The timing information isextracted by a maximum index locator 120 that locates the index of themaximum correlation value in a buffer having a size corresponding to thenumber of samples in the OFDM frame.

[0009] While the GIB algorithm performs effectively for manyapplications, it suffers from a number of limitations, which ifovercome, could greatly expand the reliability and accuracy of OFDMreceivers. For example, since each peak 210-216 in the ML metric 200occurs at the frame boundary and, in a dispersive channel, such as undermultipath conditions, the peaks will not be ideal impulses, a given peakmay start in one frame, extend over the frame boundary and end in thenext frame. Thus, a maximum correlation value associated with the peakmay be assigned an index at the end of the prior frame or the beginningof the next frame, causing ambiguities in the identification of frameboundaries.

[0010] A need therefore exists for improved techniques for performingtiming acquisition and tracking in an OFDM receiver. A further needexists for a method and apparatus for performing timing acquisition andtracking in an OFDM receiver that overcomes the problems that areinherent when the symbol time is close to the frame boundary. Yetanother need exists for a method and apparatus for performing timingacquisition and tracking in an OFDM receiver that declares when timinghas been acquired or when timing has been lost.

SUMMARY OF THE INVENTION

[0011] Generally, an improved OFDM receiver is disclosed that performstiming acquisition and tracking in a manner that overcomes theabove-described problems that are inherent when the symbol time is closeto the frame boundary. According to one aspect of the invention, theOFDM received repositions the peaks in the ML metric to a desiredposition away from the frame boundary to reduce the probability oftiming ambiguity. According to another aspect of the invention, eachOFDM frame is divided into at least two windows during an acquisitionmode in order to identify the index within each window having themaximum correlation. The improved timing acquisition of the presentinvention permits the Fast Fourier Transform (FFT) operation to operateon the correctly aligned symbol for improved accuracy. In addition, thepresent invention provides improved mechanisms for declaring when timingis acquired or when timing has been lost.

[0012] An OFDM receiver in accordance with the present inventionimplements the GIB algorithm to recover timing information from thereceived signal. The GIB timing recovery algorithm identifies peaks inthe maximum likelihood (ML) metric in a known manner. Thereafter, thepresent invention repositions each peak away from the frame boundary inorder to perform improved timing acquisition and tracking. In oneimplementation, the peaks are shifted from the frame boundary to thecenter of the OFDM frame, thereby removing the ambiguity of whether agiven peak is associated with a previous or subsequent frame. Peaks areshifted in accordance with the present invention by inserting ordeleting samples, as necessary, into each OFDM frame.

[0013] The present invention divides each OFDM frame into at least twowindows during an acquisition mode. In one implementation, each OFDMframe is divided into two windows and an index corresponding to themaximum correlation value in each window is selected, as well as anindex corresponding to the maximum correlation value in the overall OFDMframe to generate three timing estimate values, namely, tim₀, tim₁ andtim_(full). The variables tim₀ and tim₁ are the indices for the maximumcorrelation in a first window and a second window of the OFDM frame,respectively, and the variable tim_(full) indicates the index for themaximum correlation in the full OFDM frame. The timing estimates tim₀and tim₁ are evaluated during an acquisition mode to more accuratelyidentify the ML metric peak. The timing estimate tim_(full) is evaluatedduring the tracking mode to maintain the timing alignment with aspecified desired position.

[0014] According to another aspect of the invention, the timingestimates, tim₀, tim₁ and tim_(full), are applied to a timing finitestate machine (FSM) that determines when the timing information has beenacquired and shifts the OFDM signal, as necessary, to maintain the peakin the desired position during a tracking mode. For every OFDM frame,the timing FSM compares the timing estimates from the GIB algorithm withthe previous time estimates for the previous frame. If the differencesare less than the length of the guard interval consecutively for apredefined timing acquisition length, then timing acquisition iscompleted. After a predefined inter-mode settling period, the timing FSMwill transition to a tracking mode. If the differences between theestimated times and the desired position of the OFDM frame areconsistently greater than the length of the guard interval, then a lossof tracking is achieved and the timing FSM will return to an acquisitionmode.

[0015] A more complete understanding of the present invention, as wellas further features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a schematic block diagram illustrating portions of aconventional OFDM receiver directed to timing recovery;

[0017]FIG. 2 illustrates a maximum likelihood (ML) metric for an OFDMsignal;

[0018]FIG. 3 illustrates a satellite transmission system in which thepresent invention can operate;

[0019]FIG. 4 is a schematic block diagram illustrating portions of anOFDM receiver in accordance with the present invention that are directedto timing recovery;

[0020]FIGS. 5A, 5B and 5C illustrate a sliding window correlation of theGIB algorithm and the selection of the maximum values by a maximum indexlocator of FIG. 4 in accordance with one embodiment of the presentinvention;

[0021]FIG. 6 is a schematic block diagram illustrating the timingcontrol mechanism for the OFDM receiver shown in FIG. 4; and

[0022]FIG. 7 is a flow chart describing a timing FSM process implementedby the receiver shown in FIG. 4.

DETAILED DESCRIPTION

[0023]FIG. 3 illustrates a satellite transmission system 300 in whichthe present invention can operate. The satellite transmission system 300can transmit digital music or other information from an up-link station(not shown) to one or more mobile receivers, such as the mobile receiver350. As shown in FIG. 3, the illustrative satellite transmission system300 includes two satellites 310, 320 operating in a broadcast mode. Thesatellites 310, 320 are designed to be geo-stationary, and are locatedover a desired geographical coverage area, such as over the eastern andwestern United States, at appropriate angles of elevation, as dictatedby the requirements of a geo-stationary system. In one embodiment, thesatellites 310, 320 are implemented as conventional time divisionmultiplexed (TDM) transmitters.

[0024] In addition, the satellite transmission system 300 includes aplurality of terrestrial repeaters, such as the terrestrial repeater340, that will operate in dense urban areas, where the direct line ofsight (LOS) between the satellites 310, 320 and the mobile receiver 350can be blocked due to the angle of elevation and shadowing by tallbuildings. The terrestrial repeaters 340 are implemented as OFDMtransmitters to minimize the channel impairments caused by multi-pathpropagation. Although described in connection with an exemplary wirelessOFDM communication system, it will be understood that the presentinvention is equally applicable to a wired discrete multi-tone (DMT)communication system. The illustrative OFDM terrestrial repeaters 340can optionally differentially encode the transmitted signal overfrequency, as opposed to time. Thus, the differential encoding can beperformed with respect to consecutive bins (sub-carriers) in the OFDMsystem bins in order to avoid channel phase distortion. For a moredetailed discussion of an OFDM transmission system that differentiallyencodes the transmitted signal over frequency, as opposed to time, see,U.S. patent application Ser. No. 09,398,502, filed Sep. 17, 1999,entitled “Method and Apparatus for Performing Differential ModulationOver Frequency in an Orthogonal Frequency Division Multiplexing (OFDM)Communication System,” (Attorney Docket Number Riazi 3-11-3), assignedto the assignee of the present invention and incorporated by referenceherein.

[0025] The satellites 310, 320 receive the broadcast signal, e.g., froma studio, over a robust radio frequency (RF) link, and the satellites310, 320 will broadcast the signal after down-converting the signal tothe carrier frequency. The terrestrial repeaters 340 retrieve theinformation directly from an up-link studio (not shown), usingwell-known technical means, such as wireline or microwave links, or froma dedicated satellite (not shown). In the illustrative implementation,the terrestrial repeaters 340 receive the information directly from thestudio.

OFDM Signal

[0026] In the illustrative embodiment, each OFDM symbol of duration Tswill be composed of 2048 samples corresponding to the useful symbolduration and 184 samples corresponding to the guard interval, and thesymbol represents up to 2048 sub-carriers each spaced 4 kHz apart (Δf).The useful OFDM symbol duration, Tu, illustratively equals 250 mu-secand the guard interval duration or cyclic prefix duration, Tg,illustratively equals 22.46 mu-sec. The duration of the symbol, Ts, is272.46 mu-sec, where Ts equals Tu plus Tg. The inter-carrier spacing,Δf, of 4 KHz is equal to the inverse of the useful symbol duration(1/Tu).

OFDM Timing Acquisition and Tracking

[0027] According to one feature of the present invention, each OFDMreceiver repositions the peaks in the ML metric 200, such as the peaks210-216 (FIG. 2), at a desired position away from the frame boundary inorder to perform improved timing acquisition and tracking. According toanother feature of the present invention, each OFDM frame is dividedinto at least two windows in order to identify the index within eachwindow having the maximum correlation. In this manner, the OFDM receivercan properly acquire the timing so that the Fast Fourier Transform (FFT)can operate on the correctly aligned symbol. Generally, this timingestimate must be free from any ambiguity that is larger than the guardinterval duration. In addition, the timing must be properly tracked sothat the FFT receives properly aligned symbols. The tracking position ismoved from the frame boundary so that the probability of timingambiguity is minimized. In addition, the present invention providesimproved mechanisms for declaring when timing is acquired or when timinghas been lost.

[0028]FIG. 4 illustrates portions of an OFDM receiver 400 in accordancewith the present invention that are directed to timing recovery. TheOFDM receiver 400 implements the well-known Guard Interval Based (GIB)algorithm 110, as discussed above, that recovers timing information fromthe received signal. For a more detailed discussion of the GIB timingrecovery algorithm 110, see, for example, Jan-Jaap van de Beek et al.,ML Estimation of Time and Frequency Offset in OFDM Systems, IEEETransactions on Signal Processing, Vol. 45, No 7, 1800-05 (July 1997) orJan-Jaap van de Beek et al., “A Time and Frequency SynchronizationScheme for Multiuser OFDM,” IEEE J. on Selected Areas in Communications,Vol. 17, No. 11, 1900-14, (November 1999), each incorporated byreference herein.

[0029] As previously indicated, the GIB timing recovery algorithm 110identifies peaks in the maximum likelihood (ML) metric 200, shown inFIG. 2. The present invention repositions each peak, such as the peaks210-216 (FIG. 2), away from the frame boundary in order to performimproved timing acquisition and tracking. In the illustrative embodimentdescribed herein, the peaks are shifted from the frame boundary to thecenter of the OFDM frame, thereby removing the ambiguity of whether agiven peak is associated with a previous or subsequent frame. Asdiscussed further below, peaks are shifted by inserting or deletingsamples, as necessary, into each OFDM frame. As shown in FIG. 4, thetiming information is extracted from the GIB algorithm 110 by a maximumindex locator 420 that locates indices having maximum correlationvalues. For a discussion of alternatives to the GIB algorithm, see Kimet al., Performance Comparison of the Frequency Detectors for OrthogonalFrequency Division Multiplexing, IEEE Trans. Consumer Electronics, Vol.43, No. 3, 776: 783 (August 1997), incorporated by reference herein.

[0030] In the illustrative embodiment, each OFDM frame is divided intotwo windows and an index corresponding to the maximum correlation valuein each window is selected, as well as an index corresponding to themaximum correlation value in the overall OFDM frame. Thus, as shown inFIG. 4, the illustrative maximum index locator 420 generates threetiming estimate values, namely, tim₀, tim₁ and tim_(full). The variablestim₀ and tim₁ are the indices for the maximum correlation in a firstwindow and a second window of the 2232-sample buffer, respectively.Finally, the variable tim_(full) indicates the index for the maximumcorrelation in the 2232-sample buffer.

[0031] The timing estimates, tim₀, tim₁ and tim_(full), are applied to atiming finite state machine (FSM) 450, discussed below in conjunctionwith FIGS. 6 and 7, that determines when the timing information has beenacquired and shifts the OFDM signal, as necessary, to maintain the peakin the desired position during a tracking mode. In the illustrativeembodiment, each peak is maintained in the center position of the OFDMframe (sample position 1116). As discussed further below, the timing FSM450 provides reliable transitions between the acquisition and trackingmodes of operation.

[0032] Generally, during the acquisition mode, the timing FSM 450 setsthe variable acquisition status, ACQSTAT, to a binary value of one (1)for one frame when the delete/add stage 630 (FIG. 6, discussed below) isactive. In addition, the timing FSM 450 aligns the peak of thecorrelated OFDM signal with the desired position of the OFDM frame,DesiredPos (the central position, 1116, in the illustrative embodiment)using the variable ACQTIME. The desired position is selected such thatafter acquisition the ideal timing instant will be positioned away fromthe frame boundary, thus minimizing edge ambiguities. For every OFDMframe, the timing FSM 450 also compares the time estimates from the GIBalgorithm 110 with the previous time estimates for the previous frame.If the differences are less than the length of the guard intervalconsecutively for a predefined timing acquisition length, TacqLen, suchas five (5) frames in the illustrative embodiment, then timingacquisition is completed. After a predefined inter-mode settling period,the timing FSM will transition to a tracking mode.

[0033] Similarly, during the tracking mode, the timing FSM 450 sets thevariable tracking status, TRACKSTAT, to a binary value of one (1). Whilein the tracking mode, the timing FSM 450 compares the estimated timeswith the desired position of the OFDM frame, DesiredPos (sample position1116). For every OFDM frame, the timing FSM 450 also compares the timeestimates from the GIB algorithm 110 with the previous time estimatesfor the previous frame. The peak of the correlated OFDM signal ismaintained in the desired position of the OFDM frame, DesiredPos, usingthe variable TRACKTIME. If the differences between the estimated timesand the desired position of the OFDM frame are consistently greater thanthe length of the guard interval, then a loss of tracking is achievedand the timing FSM 450 will return to an acquisition mode.

[0034]FIGS. 5A, 5B and 5C illustrate the sliding window correlation ofthe GIB algorithm 110, and the selection of the maximum values by themaximum index locator 420 in accordance with an illustrative embodimentof the present invention. As previously indicated, the samples processedby the GIB algorithm 110 are heavily correlated at a lag of the usefulsymbol duration (2048 samples in the illustrative implementation). Thiscorrelation is accomplished by the moving average blocks in the GIBalgorithm 110 that generate the sum of the most recent 184 samples fedto the moving average blocks (not shown). The timing information isextracted by locating indices of the maximum correlation value invarious windows of a buffer that is 2232 samples wide.

[0035] As shown in FIG. 5A, each OFDM frame 500 consisting of 2232samples in the illustrative embodiment (2048 active samples, and a guardinterval of 184 samples), is divided into a first window 510 and asecond window 520. The timing estimate tim₀ is the index for the maximumcorrelation in the first window 510 of the 2232-sample buffer. Thetiming estimate tim₁ is the index for the maximum correlation in thesecond window 520 of the 2232-sample buffer. Finally, the timingestimate tim_(full) indicates the index for the maximum correlation infull 2232-sample buffer corresponding to the entire OFDM frame. FIG. 5Billustrates the correlation of the 184 guard interval samples 531, 532in two subsequent OFDM frames. Each 184 guard interval sample 531, 532is 2232 samples apart. The correlation process is repeated to get the2232 sample correlation output, as shown in FIG. 5C.

[0036]FIG. 6 is a schematic block diagram illustrating the timingcontrol mechanism for the OFDM receiver 400 shown in FIG. 4. As shown inFIG. 6, the OFDM receiver 400 receives an OFDM baseband signal 610 at anillustrative rate of 30 Msamples/second. The interpolation block 620uses the time tracking signal generated by the timing FSM 450 during thetracking mode to simultaneously adjust the timing (maintain alignmentwith desired position, DesiredPos) and down-sample the signal to twicethe oversampling rate. During the acquisition mode, the value of theTRACKSTAT value is 0, so the interpolation block 620 is inactive. Duringthe tracking mode, however, the value of the TRACKSTAT value is 1, sothe interpolation block 620 serves to shift the peak in accordance withthe number of samples indicated by the variable, TRACKTIME.

[0037] The add/delete block 630 is used during the acquisition mode onlyfor acquisition or re-acquisition purposes. The number of samples thatare added to or deleted from the sample stream by the add/delete block630 is dictated by the timing estimate after the acquisition iscomplete, discussed below, in accordance with the variable ACQTIME.During the tracking mode, the value of the ACQSTAT variable is 0, so theadd/delete block 630 is inactive. During the acquisition mode, however,the value of the ACQSTAT value is 1 for one frame while the add/deleteblock 630 is active, so the add/delete block 630 serves to shift thepeak to the desired position, DesiredPos, in accordance with the numberof samples indicated by the variable, ACQTIME. The following tablesummarizes the values of the status bits during the various operatingmodes: ACQSTAT TRACKSTAT Acquisition 1 (for 1 frame, otherwise 0) 0 ModeInter-Mode 0 0 Settling Period Tracking 0 1 Mode

[0038]FIG. 7 is a flow chart describing the timing FSM process 700implemented by the receiver 400 shown in FIG. 4. As shown in FIG. 7, thetiming FSM process 700 initially performs a test during step 710 todetermine if the timing estimate, tim0, varies by less than the guardinterval from frame to frame for a predefined number (tacqlen) offrames. If it is determined during step 710 that the timing estimate,tim0, varies by less than the guard interval from frame to frame for apredefined number (tacqlen) of frames then the timing estimate, tim0, isthe correct timing, the variable ACQTIME is established as thedifference between the timing estimate, tim0, and the desired position,DesiredPos, and the status bit ACQSTAT is set to one (indicating theacquisition mode) during step 730.

[0039] If, however, it is determined during step 710 that the timingestimate, tim0, does not vary by less than the guard interval for apredefined number (tacqlen) of frames then a further test is performedduring step 720 to determine if the timing estimate, tim1, varies byless than the guard interval for a predefined number (tacqlen) offrames. If it is determined during step 720 that the timing estimate,tim1, varies by less than the guard interval for a predefined number(tacqlen) of frames then the timing estimate, tim1, is the correcttiming, the variable ACQTIME is established as the difference betweenthe timing estimate, tim1, and the desired position, DesiredPos, and thestatus bit ACQSTAT is set to one (indicating the acquisition mode)during step 740.

[0040] The variable tim_(full) indicating the index for the maximumcorrelation in the 2232-sample buffer, is determined during step 750.Thereafter, the timing FSM process 700 waits for a predefined number offrames during step 760 to permit the timing FSM 450 to settle. Thevariable tim_(full) is established as the timing estimate during step770. A test is performed during step 780 to determine if the timingestimate, timfull, varies by more than the guard interval from thedesired position, DesiredPos, for a predefined number (tacqloslen) offrames. If it is determined during step 780 that the timing estimate,timfull, varies by more than the guard interval from the desiredposition, DesiredPos, for a predefined number (tacqloslen) of frames,then tracking is lost and the status bit TRACKSTAT is set to a binaryvalue of zero (0) during step 790 and program control returns to step710 to reacquire timing.

[0041] If, however, it is determined during step 950 that the timingestimate, timfull, does not vary by more than the guard interval fromthe desired position, DesiredPos, for a predefined number (tacqloslen)of frames, then the OFDM frame is realigned with the desired position,DesiredPos, if necessary, and the status bit TRACKSTAT is set to abinary value of one (1) to maintain the timing FSM 450 in the trackingmode during step 785.

[0042] It is to be understood that the embodiments and variations shownand described herein are merely illustrative of the principles of thisinvention and that various modifications may be implemented by thoseskilled in the art without departing from the scope and spirit of theinvention.

I claim:
 1. A method for processing a signal in a communication system,said method comprising the steps of: receiving said signal; correlatingsaid signal with itself to identify a peak in each frame; and aligningsaid peak with a desired position within said frame, said desiredposition being removed from a frame boundary.
 2. The method according toclaim 1, wherein said desired position is a central position in saidframe.
 3. The method according to claim 1, further comprising the stepof establishing a plurality of windows in each frame during anacquisition mode and determining a peak in each of said windows.
 4. Themethod according to claim 3, further comprising the step of selecting atiming estimate associated with a peak in one of said windows for timingpurposes if said timing estimate satisfies at least one predefinedcondition.
 5. The method according to claim 4, wherein said at least onepredefined condition ensures that said timing estimate does not vary bymore than a guard interval for a predefined number of frames.
 6. Themethod according to claim 5, further comprising the step of declaringthat timing has been acquired when said at least one predefined criteriais satisfied.
 7. The method according to claim 1, further comprising thestep of declaring that timing has been lost during a tracking mode whensaid peak varies from said desired position by more than a guardinterval in said signal for more than a predefined number of frames. 8.The method according to claim 1, wherein said correlating stepdetermines the maximum likelihood (ML) metric of said signal.
 9. Themethod according to claim 1, wherein said correlating step is performedat a lag of the useful symbol duration.
 10. The method according toclaim 1, wherein said signal is an OFDM signal.
 11. The method accordingto claim 1, wherein said signal is a DMT signal.
 12. A method forprocessing a signal in a communication system, said method comprisingthe steps of: receiving said signal; identifying a peak in each frame;and aligning said peak with a desired position within said frame, saiddesired position being removed from a frame boundary.
 13. The methodaccording to claim 12, wherein said desired position is a centralposition in said frame.
 14. The method according to claim 12, furthercomprising the step of establishing a plurality of windows in each frameduring an acquisition mode and determining a peak in each of saidwindows.
 15. The method according to claim 14, further comprising thestep of selecting a timing estimate associated with a peak in one ofsaid windows for timing purposes if said timing estimate satisfies atleast one predefined condition.
 16. The method according to claim 15,wherein said at least one predefined condition ensures that said timingestimate does not vary by more than a guard interval for a predefinednumber of frames.
 17. The method according to claim 16, furthercomprising the step of declaring that timing has been acquired when saidat least one predefined criteria is satisfied.
 18. The method accordingto claim 12, further comprising the step of declaring that timing hasbeen lost during a tracking mode when said peak varies from said desiredposition by more than a guard interval in said signal for more than apredefined number of frames.
 19. The method according to claim 12,wherein said correlating step determines the maximum likelihood (ML)metric of said signal.
 20. The method according to claim 12, whereinsaid identifying step further comprises the step of correlating saidsignal with itself at a lag of the useful symbol duration to identify apeak in each frame step.
 21. A receiver for receiving a signal,comprising: a maximum index locator to identify a peak in each frame;and an alignment block for aligning said peak with a desired positionwithin said frame, said desired position being removed from a frameboundary.
 22. The receiver according to claim 21, wherein said desiredposition is a central position in said frame.
 23. The receiver accordingto claim 21, wherein a peak is identified in a plurality of windows ineach frame during an acquisition mode.
 24. The receiver according toclaim 23, wherein a timing estimate associated with a peak in one ofsaid windows is selected for timing purposes if said timing estimatesatisfies at least one predefined condition.
 25. The receiver accordingto claim 24, wherein said at least one predefined condition ensures thatsaid timing estimate does not vary by more than a guard interval for apredefined number of frames.
 26. The receiver according to claim 25,wherein timing is acquired when said at least one predefined criteria issatisfied.
 27. The receiver according to claim 21, wherein timing islost during a tracking mode when said peak varies from said desiredposition by more than a guard interval in said signal for more than apredefined number of frames.
 28. The receiver according to claim 21,wherein said maximum index locator determines the maximum likelihood(ML) metric of said signal.
 29. The receiver according to claim 21,wherein said maximum index locator operates at a lag of the usefulsymbol duration.
 30. The receiver according to claim 21, wherein saidsignal is an OFDM signal.
 31. The receiver according to claim 21,wherein said signal is a DMT signal.